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Transcript 
Chapter 19
Plate Tectonics
Alfred Wegener
 A man
with an idea.
 In 1912
 Noticed similar shapes
 similar fossils
 Similar rock structures
 Similar evidence from glaciers
 on continents
Problems With the Theory
 Though
winds or ocean currents could
explain the fossils
 Thought the poles moved, so that you
could get the glaciers
 Didn’t explain how it happened.
 Though that it was due to the Earth
spinning on its axis
 Scientists in the southern hemisphere
accepted it first
 They believed the rocks
Solution
 Arthur
Holmes- 1928
 Convection currents move the plates.
Ocean-Floor Spreading
 In
the 1950’s and 1960’s mapped the
ocean floor
 Examined underwater mountain ranges
 Run for 37,500 miles around the earth.
 Found a rift valley in the center
 Harry Hess - 1962 explained
– new ocean floor is made at the midocean ridge
– Is pushed under other crust at the
trenches
Evidence trapped in rocks
 Earth
is a magnet
 When lava cools it keeps track of which
way the magnetic pole points.
 Every few hundred thousand years the
magnetic field switches direction.
 Has made stripes of alternating directions
on the ocean floor.
Other Evidence
 By
drilling they have found that rocks
near the ridge are youngest, and they get
older as you get further away.
Where does it go?
 At
the edges of the plates one goes under
the other
 called subduction
 It is melted and may make volcanoes.
 Evidence comes from earthquakes.
 The further you get from the edge, the
deeper earthquakes are.
Earthquakes and Volcanoes
 Occur
in zones around the world
Lithospheric Plates
 There
others
are 7 major ones,and several
Parts of the lithosphere
Iceland
Convergent Boundary
Convergent Boundary
Convergent Boundary
Convergent Boundaries
 Continental
and oceanic- ocean
subducted, volcanoes on shore
 Oceanic and Oceanic- one is subducted,
forms an island arc
 Continental and continental- folded
mountains and a plateau
Boundary types
How Fast?
 Arctic
Ridge 2.5 cm per year
 East Pacific rise, off the coast of Chile 15
cm/year
 Current movement tracked by satellites
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